Control valve for a hydraulic power steering system

ABSTRACT

A control valve for a hydraulic power steering system includes a pair of flow circuits arranged in parallel. Each flow circuit includes an upstream and downstream flow orifices arranged in series. Each set of orifices defines between the orifices an intermediate pressure zone of the valve. Both orifices close together from a centered condition of the valve and fully close substantially together at a fully closed condition of the valve. The downstream orifice is larger than the upstream orifice when open to minimize the likelihood of cavitation and noise. An equalization passage fluidly connects both intermediate pressure zones to equalize pressure despite variations in orifice size due to manufacturing tolerances. The control valve includes a bypass passage in parallel with the flow circuits. A pressure relief valve in the control valve has a normally closed valving member in the bypass passage that opens the valve in the event of overpressure.

FIELD OF THE INVENTION

The invention relates to a hydraulic power steering system for turningthe steerable wheels of a vehicle, and more particularly, to a controlvalve for controlling the flow of hydraulic fluid to a hydraulic motorof the power steering system.

DESCRIPTION OF THE PRIOR ART

Trucks and automobiles are commonly provided with a hydraulic powersteering system to turn the steerable wheels of the vehicle. The driverturns the steering wheel and the power steering system generates theforce to turn the wheels.

A typical hydraulic power steering system includes a pump that flowshigh-pressure hydraulic fluid to a hydraulic motor. The hydraulic motorhas a movable piston that drives an output member connected to thevehicle steering linkage. The wheels are mounted to the steeringlinkage.

Movement of the piston drives the output member to turn the wheels leftor right. For a right turn, fluid flows to one side of the piston anddrives the piston in a direction that turns the wheels right. For a leftturn, fluid flows to the other side of the piston and drives the pistonin the opposite direction that turns the wheels left.

The flow between the pump and hydraulic motor is controlled by a controlvalve. The control valve receives flow from the pump and controls theflow to the hydraulic motor. Operating the control valve directs theflow of fluid to one side of the piston or the other, and controls theamount of flow to the piston.

The control valve is connected to the steering wheel such that turningthe steering wheel operates the control valve. During straight-aheaddriving the steering wheel is centered. The control valve is in acentered condition and the wheels are straight. If the steering wheel isturned right, the control valve is placed in an off-center conditionthat flows fluid to one side of the piston. The wheels turn right. Ifthe steering wheel is turned left, the control valve is placed in anoff-center condition that flows fluid to the other side of the piston.The wheels turn left.

Operating the control valve closes a flow restriction or flow orifice inthe control valve. The orifice directs the flow of fluid to one side ofthe piston or the other. The orifice forces some fluid to flow to thepiston and move the piston. The remaining fluid flows through theorifice and returns to the pump. The orifice is a variable-flow orificethat closes with increasing steering wheel rotation. This flows morefluid to the piston and provides increasing power assist for the driver.

Fluid flowing through the control valve, however, may generate noise. Acommon cause of noise is cavitation of the fluid flowing through theorifice. Cavitation is caused by bubbles forming in the fluid as aresult of the static fluid pressure dropping as the fluid flows throughthe orifice. If the pressure drop is great enough, the bubbles form andimplode as the fluid is discharged from the orifice. The implosionsgenerate noise that may be heard in the vehicle passenger compartment.

The likelihood that a particular fluid will cavitate is given by theCavitation Number C_(a):C _(a)=(P _(a) −P _(v))*A ²/(½*p*Q ²) where

-   -   P_(a)=static fluid pressure at the orifice discharge;    -   P_(v)=vapor pressure of the hydraulic fluid;    -   A=cross-sectional area of the orifice;    -   p=mass density of the hydraulic fluid; and

Q=volumetric flow rate of hydraulic fluid through the orifice.

The higher the Cavitation Number the less likely it is that cavitationwill occur. Cavitation Number increases for a given flow rate Q withincreasing orifice area A and increasing discharge pressure. Thecritical Cavitation Number above which flow will usually not cavitate istypically between 0.2 and 1.5 for hydraulic fluids.

As truck cabs and automobile interiors have become quieter, demands havebeen placed on power steering manufacturers to reduce power steeringnoise.

Reducing cavitation can reduce power steering noise. To reducecavitation, some control valves include two orifices arranged in serieswithin the valve. Each orifice can be larger than a single, sole orificeto flow the same volume of fluid. The increased orifice area increasesthe Cavitation Number for each orifice and reduces the likelihood ofcavitation. Furthermore, the upstream orifice has a greater dischargepressure that further increases its Cavitation Number.

Although placing two orifices in series may increase Cavitation Number,known control valves do not configure the two orifices to best minimizenoise.

In one known control valve, both upstream and downstream orifices closebut remain identically sized with one another. The Cavitation Number ofthe upstream orifice is always higher than the Cavitation Number of thedownstream orifice. This is caused by the discharge pressure of theupstream orifice always being greater than the discharge pressure of thedownstream orifice. The downstream orifice is more likely to cavitatethan the upstream orifice. This limits the overall effectiveness of thetwo orifices to reduce noise.

In another known control valve the downstream orifice closes to aminimum area and maintains that minimum area with further rotation ofthe steering wheel. As the upstream orifice continues to close, theentire pressure drop effectively occurs through the upstream orifice.The upstream orifice essentially functions as a sole orifice. Thebenefit of having two orifices in series to reduce noise is lost.

In yet another known control valve the downstream orifice is alwayssmaller than the upstream orifice. The Cavitation Number of thedownstream orifice, therefore, is decreased as compared to having twoequally-sized orifices. Accordingly the downstream orifice is morelikely to cavitate. Hence the downstream orifice limits theeffectiveness of the two orifices to reduce noise.

Additionally, having two orifices in series can affect reliability ofthe control valve. Variations in orifice size caused by manufacturingtolerances may cause the steering wheel to “stick” as the driverattempts to turn the steering wheel.

Conventional control valves have two or more fluid circuits that flowfluid within the valve. Each fluid circuit includes a set of upstreamand downstream orifices. The fluid circuits extend around or along amovable inner valve member connected to the steering column. Thecircuits are arranged symmetrically so that no side forces are generatedagainst the valve member.

Variations in orifice sizes caused by manufacturing tolerances, however,generate a load imbalance that applies a side force against the valvemember. Power steering systems for heavy duty trucks operate atsufficiently high pressures that this side force may cause “stiction”,or resistance to valve movement, that is perceived by the driver assticking of the steering wheel during turning.

Furthermore, control valves are typically mounted in a bearing cap orthe like that forms part of the power steering system. The bearing capmay mount a pressure relief valve fluidly connected in parallel with thecontrol valve. The relief valve prevents a fluid overpressure from beingdelivered to the hydraulic motor.

The location of the relief valve in the bearing cap can vary accordingto vehicle manufacturers' requirements, and some manufacturers eliminatethe relief valve entirely. A number of otherwise identical bearing capsmust be manufactured and kept in inventory to satisfy different vehiclemanufacturers' requirements.

It would be desirable that a quiet and reliable control valveincorporate a pressure relief valve to reduce the different types ofbearing caps that must be kept in inventory, thereby reducing theoverall cost of the power steering systems.

Therefore there is a need for an improved control valve for reducingnoise in hydraulic power steering systems. The improved control valveshould be reliable under heavy duty operating conditions and shouldincorporate a pressure relief valve to lower inventory costs.

SUMMARY OF THE INVENTION

The present invention is an improved control valve that reduces noise inhydraulic power steering systems. The control valve is reliable underheavy duty conditions and can include a pressure relief valve to lowerinventory costs.

A control valve in accordance with the present invention includes firstand second orifices in series in a flow path. The first and secondorifices each close with operation of the control valve from thecentered condition. The orifices close to an essentially fully closedcondition at about the same movement of the control valve from thecentered condition to a closed condition of the control valve.

The second orifice is downstream from the first orifice and has asubstantially larger flow area than the first orifice when the first andsecond orifices are not fully closed.

Maintaining the area of the downstream orifice larger than the upstreamorifice when the orifices are open maximizes noise reduction. Asdiscussed above, having the two orifices equal in size results in theupstream orifice always having a higher Cavitation Number than thedownstream orifice. By reducing the upstream orifice and enlarging thedownstream orifice, the Cavitation Number of the upstream orificedecreases and the Cavitation Number of the downstream orifice increases.

The relative sizes of the upstream and downstream orifices are selectedto best match the Cavitation Numbers of the two orifices and reduce theoverall likelihood of cavitation. Specific sizes of the upstream anddownstream orifices are preferably determined based on the flow rate andpressure requirements of the specific power steering systemincorporating the control valve.

Closing both orifices together ensures that the total pressure drop iseffectively shared by both orifices even as the orifices approach thefully closed condition. This helps minimize noise when power assist isthe greatest and the likelihood of cavitation is greatest.

A preferred embodiment of the control valve includes at least two fluidcircuits, each circuit having a set of upstream and downstream orifices.Each set of orifices defines an intermediate pressure zone between them.An equalization passage fluidly connects the intermediate pressure zonesand ensures that the pressures in the intermediate pressure zones areequal. This prevents stiction that would otherwise be caused byvariations in orifice sizes due to manufacturing tolerances. Thereliability of the control valve under heavy duty operating conditionsis increased.

In yet other embodiments of the control valve the intermediate pressurezone includes at least one additional flow restriction that generatesadditional pressure drop. The sum of the pressure drop through theupstream and downstream orifices is reduced by the pressure drop throughthe intermediate zone, thereby reducing the likelihood of cavitation andnoise. These additional flow restrictions can be formed by changing thedirection of flow between the upstream and downstream orifices, or byplacing additional orifices between the upstream and downstreamorifices.

In additional embodiments of the control valve, the control valveincludes a pressure relief valve that prevents a fluid overpressure frombeing delivered to the hydraulic motor. The pressure relief valveincludes a bypass passage in the control valve extends between the valveinlet and exhaust in parallel with the flow path. A movable valvingmember in the bypass passage is held in a normally-closed position thatcloses the bypass passage. A fluid overpressure moves the valving memberto open the bypass passage and relieve the overpressure condition. Theoperating characteristics of the pressure relief valve can be adapted todifferent power steering systems without modifying the bearing cap orother components of the system.

Other objects and features of the invention will become apparent as thedescription proceeds, especially when taken in conjunction with theaccompanying 17 drawing sheets illustrating three embodiments of theinvention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is representational view of a hydraulic power steering systemhaving a first embodiment control valve in accordance with the presentinvention;

FIG. 2 is a sectional view of the power steering gear takensubstantially along line 2—2 of FIG. 1;

FIG. 3 is an exploded view of the inner and outer valve members of thefirst embodiment control valve;

FIG. 4 is a schematic sectional view of the first embodiment controlvalve, the control valve in a centered condition;

FIG. 5 is an enlarged view of a portion of FIG. 4;

FIG. 6 is a view similar to FIG. 5 but with the control valve in anoff-center condition;

FIGS. 7 a, 7 b and 7 c are enlarged views of the closing orifices shownin FIG. 6;

FIG. 8 is a valve curve for the control valve plotting the areas of theclosing flow orifices in the control valve against the relative angularrotation of the valve core with respect to the valve sleeve;

FIG. 9 is a representational hydraulic diagram of the fluid circuits inthe first embodiment control valve with the control valve in theoff-center condition;

FIG. 10 is a representational view of the fluid circuits extendingaround the valve core of the control valve when the valve core isrotated in a first direction;

FIG. 11 is a view similar to FIG. 10 but with the valve core rotated ina second direction;

FIG. 12 is an exploded view of a second embodiment control valve inaccordance with the present invention;

FIG. 13 is a top view of the second embodiment control valve;

FIG. 14 is a sectional view taken along lines 14—14 of FIG. 13;

FIG. 15 is a side view of the control valve shown in FIG. 13;

FIG. 16 is a sectional view taken along lines 16—16 of FIG. 13;

FIG. 17 is a side view of a third embodiment control valve in accordancewith the present invention;

FIG. 18 is a sectional view taken along line 18-18 of FIG. 17, thecontrol valve in an off-center condition;

FIG. 19 is a perspective view of the valve core of the control valveshown in FIG. 17;

FIG. 20 is a sectional view similar to FIG. 18 but taken along line20—20 of FIG. 17; and

FIG. 21 is a sectional view similar to FIG. 18 but taken along line21—21 of FIG. 17.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 illustrates a first embodiment control valve 10 in accordancewith the present invention. The control valve 10 forms part of ahydraulic power steering system 12 that turns the steerable wheels of amotor vehicle. The steering system 12 includes a pump 14 that flowspower steering fluid from fluid reservoir 16 to a hydraulic motor 18formed in high-pressure steering gear 20. Steering gear 20 could be anM-Series steering gear manufactured by R.H. Sheppard Company, Inc.,Hanover, Pa., assignee of the present invention. Hydraulic motor 18drives a Pitman arm 22 connected to the vehicle steering linkage to turnthe wheels.

Control valve 10 is mounted in steering gear 20 and controls the flow offluid from the pump 14 to the hydraulic motor 18. Supply line 24 flowsfluid from the pump 14 to the control valve 10 and return line 26 flowsfluid from the control valve 10 to the fluid reservoir 16.

Steering gear 20 includes a housing 28 that defines a chamber 30. SeeFIG. 2. Bearing cap 32 closes one end of the chamber and cylinder head34 closes the other end of the chamber. Hydraulic motor 18 includesaxially-movable motor piston 36 that sealingly divides the chamber 30into first and second hydraulic motor chambers 38, 40 on either side ofthe piston. First motor chamber 38 is adjacent the bearing cap andsecond motor chamber 40 is adjacent the cylinder head. Toothed rack 42formed on one side of the piston engages sector gear 44 attached to anoutput shaft 46 that extends into the housing. Pitman arm 22 is mountedon the output shaft 46 away from the steering gear 20.

Control valve 10 is supported in the bearing cap 32. The bearing cap 32includes an inlet passage 48 that flows fluid from the supply line 24 tothe control valve, and an outlet passage 50 that flows fluid from thecontrol valve to the return line 26. First motor chamber passage 52extends through the bearing cap to flow fluid between the control valve10 and the first motor chamber 38. Second motor chamber passage 54extends through the bearing cap and the gear housing 28 to flow fluidbetween the control valve 10 and the second motor chamber 40.

Bearing cap 32 functions as a housing for control valve 10 and housesouter valve member or valve sleeve 56 and inner valve member or valvecore 58. Valve core 58 is coaxially mounted in the valve sleeve 56 forrotation about an axis of rotation. The valve core 58 has limitedrotation from a centered position with respect to the valve sleeve 56 tooperate the control valve. An end 60 of the valve core 58 extends out ofthe bearing cap 32 and is configured in a conventional manner forconnection to a steering column that carries the steering wheel of themotor vehicle.

A co-axial tubular actuating shaft 62 extends from the valve sleeve 56and engages a conventional recirculating ball assembly 64 carried in thepiston 36. The actuating shaft 62 has a threaded outer surface thatthreadingly engages the balls of the ball assembly 64. Actuating shaft62 and valve sleeve 56 are preferably formed as a one-piece integralmember 66 (see also FIG. 3).

A torsion bar 68 within actuating shaft 62 is connected between theactuating shaft 62 and the valve core 58. The torsion bar biases thevalve members 56, 58 to the centered condition of the valve 10. One endof the torsion bar is pinned to the actuating shaft 62. The other end ofthe torsion bar extends into a blind bore 70 on the inner end of thevalve core 58 and is pinned to the valve core 58. Rotation of the valvecore 58 from the centered position twists the torsion bar and generatesa biasing force that urges the valve members 56, 58 back to the centeredposition.

Turning the steering wheel operates the control valve 10 and rotates thevalve core 58 from a centered or non-steering position. This changes thecontrol valve 10 from a centered condition to an off-center condition.This flows high-pressure hydraulic fluid from the control valve 10through the first or second motor passages 52, 54 to one of the motorchambers 38, 40 on one side of the piston 36. Piston 36 then moves leftor right and forces fluid out of the other motor chamber 40, 38. Fluidreturning from the other motor chamber flows back to the control valve10 through the other of the first or second motor passages 54, 52.

Piston movement causes rack 42 to drive the sector gear 44 and rotatesPitman arm 22 to turn the wheels left or right. Piston movement alsocauses the recirculating ball assembly 64 to rotate the actuating shaft62 and the valve sleeve 56 relative to the valve core 58. Valve core 58returns to a centered position with respect to valve sleeve 56 when thewheels have turned to the extent corresponding to the turning of thesteering wheel. This returns control valve 10 to a centered condition ina conventional manner.

FIG. 3 illustrates the valve core 58 and member 66. Valve sleeve 56 hasa generally cylindrical outer periphery 72 and a generally cylindricalinner periphery 74 (see FIG. 2). Valve core 58 has a generallycylindrical outer periphery 76 that cooperates with the inner peripheryof valve sleeve 56 to define an internal flow channel 78 (see FIGS. 2and 4) between the valve members 56, 58.

Outer periphery 72 of valve sleeve 56 includes a number ofaxially-spaced annular grooves 80, 82, 84, and 86. The grooves cooperatewith the bearing cap 32 to define flow passages flowing fluid into andout of the control valve 10. Inlet groove 80 is aligned with bearinginlet passage 48 and receives fluid flowing into the control valve fromthe pump 14. Outlet groove 82 is aligned with bearing outlet passage 50and discharges fluid from the control valve 10 to the reservoir 16.First motor chamber groove 84 is aligned with first motor passage 52 andflows fluid between the control valve 10 and the first motor chamber 38.Second motor chamber groove 86 is aligned with second motor passage 54and flows fluid between the control valve 10 and the second motorchamber 40.

FIG. 4 illustrates the control valve 10 in the centered or neutralcondition. Valve core 58 is in a centered position with respect to valvesleeve 56. Flow passage 78 surrounds the outer periphery of the valvecore. The upper half of flow passage 78 as shown in FIG. 4 forms aportion of a first flow passage or fluid circuit 88, and the lower halfforms a portion of a like second flow passage or fluid circuit 90. Thefluid circuits 88, 90 are arranged in parallel to direct the flow offluid between flow passage 78 and the motor chambers 38, 40 as will bediscussed in greater detail below.

Valve sleeve 56 includes sets of ports that are each associated with arespective fluid circuit 88 or 90 to connect the circuit with theannular flow passages 80–86. Each set of ports include an inlet port 92a or 92 b that flows fluid from inlet flow passage 80 into the circuitand a pair of exhaust ports 94 a and 94 b located ninety degrees fromcorresponding inlet port 92 a or 92 b. The exhaust ports 94 a, 94 b areshared by both fluid circuits and discharge fluid from the fluidcircuits into outlet passage 82. First motor port 96 a or 96 b islocated on one side of corresponding inlet port 92 a or 92 b and flowsfluid between the fluid circuit and the first motor chamber groove 84.Second motor port 98 a or 98 b is located on the other side of thecorresponding inlet port 92 and flows fluid between the fluid circuitand the second motor chamber groove 86.

Valve sleeve 56 has eight lands 100 a, 100 b, 100 c, 100 d, 102 a, 102b, 102 c, and 102 d that extend axially and are spaced apartcircumferentially on inner periphery 74. Lands 100, 102 definecircumferentially-spaced apart axial grooves 104 a, 104 b, 104 c, 106 a,106 b, 106 c, 108 a, and 108 b between alternate lands.

Lands 100 and grooves 104 are associated with the first fluid circuit88, lands 102 and grooves 106 are associated with the second fluidcircuit 90, and grooves 108 are shared between the circuits. Inlet ports92 a and 92 b open into respective grooves 104 a and 106 a and exhaustports 94 a and 94 b open into respective grooves 108 a and 108 b. Firstmotor ports 96 a and 96 b extend through respective lands 100 a and 102a and second motor ports 98 a and 98 b extend through respective lands100 b and 102 b.

Valve core 58 has eight lands 110 a, 110 b, 110 c, 112 a, 112 b, 112 c,114 a, and 114 b that extend axially and are spaced apartcircumferentially on outer periphery 76. Lands 110, 112, and 114 definecircumferentially-spaced apart axial grooves 116 a, 116 b, 116 c, 116 d,118 a, 118 b, 118 c, and 118 d between alternate lands. Lands 110 andgrooves 116 are associated with the first fluid circuit 88, and lands112 and grooves 118 are associated with the second fluid circuit 90.

Inlet ports 92 a and 92 b face respective core lands 110 a and 112 a.Exhaust ports 94 a and 94 b face respective core lands 114 a and 114 b.First motor ports 96 a and 96 b face respective core grooves 116 a and118 a. Second motor ports 98 a and 98 b face respective core grooves 116b and 118 b.

A pair of axially-spaced equalization passages 120 and 122 fluidlyconnect respective pairs of core grooves 116 d, 118 d and 116 c, 118 c.Each passage 120, 122 is entirely within the valve core 58 and is formedas a through-bore extending along a diameter of the valve core. Thefunction of equalization passages 120, 122 will be explained in greaterdetail later.

Valve sleeve 56 and valve core 58 are each symmetrical about a commoncentral axial plane 140 when control valve 10 is in the neutralcondition. Core land 110 a is centered in sleeve groove 104 a, and corelands 114 a and 114 b are centered in sleeve grooves 108 a and 108 brespectively. Core lands 110 b and 110 c, however, are each offsettowards the side of adjacent sleeve land 100 b or 100 a. It isunderstood that the corresponding core and sleeve lands in fluid circuit90 are arranged in like manner.

Valve core lands 110, 112 and 114 cooperate with valve sleeve lands 100,102 to define variably-sized restrictions or variable flow orifices inthe fluid circuits 88, 90. FIG. 5 illustrates flow orifices 124-136 inthe upper fluid circuit 88, it being understood that the followingdiscussion also applies in like manner to the lower fluid circuit 90.

Orifice 124 is between pump inlet 92 a and second motor port 98 a and isdefined by core land 110 a and sleeve land 100 b. Orifices 126 and 128are both between first motor port 96 a and exhaust port 94 a. Orifice126 is defined by core land 110 c and sleeve land 100 a and orifice 128is defined by core land 114 a and sleeve land 100 c.

Orifice 130 is between inlet port 92 a and first motor port 96 a and isdefined by core land 110 a and sleeve land 100 a. Orifices 132 and 134are both between second motor port 98 a and exhaust port 94 b. Orifice132 is defined by core land 11 b and sleeve land 100 b, and orifice 134is defined by core land 114 b and valve land 100 d.

As shown in FIG. 5, orifices 124-134 are each open when valve core 58 isin the centered position. The size of each orifice is essentiallyestablished by the circumferential spacing between associated pairs ofsleeve and core lands defining the orifice.

Fluid from the pump 14 enters fluid circuit 88 through motor port 92 a.Circuit 88 is symmetrical about motor port 92 a, with inlet port 92 adividing fluid circuit 88 into a first flow passage 136 extending frominlet port 92 a to exhaust port 94 a and a second flow passage 138extending from inlet port 92 a to exhaust port 94 b.

Orifice 124 is located at the entrance to second flow passage 138, andorifice 130 is located at the entrance to first flow passage 136.Orifices 124 and 130 are referred to herein as pressure orifices becausethey affect pressurization of the motor ports as will be explained ingreater detail below. Pairs of orifices 126, 128 and 130, 132 arearranged in series in respective flow passages to flow fluid to exhaust.Orifice 126 is upstream of orifice 128 in flow passage 136, and orifice132 is upstream of orifice 134 in flow passage 138. Because of theirrelative locations, orifices 126 and 132 are referred to herein asupstream exhaust orifices and orifices 128 and 134 as downstream exhaustorifices.

Flows through passages 136 and 138 are essentially balanced andsubstantially equal when the control valve 10 is centered. The fluidpressure at first motor port 96 a is effectively the same as the fluidpressure at second motor port 98 a.

Fluid flow through fluid circuit 90 is balanced in the same way as fluidcircuit 88. As a result, fluid pressures in motor chambers 38 and 40 aresubstantially equal and the piston 36 is stationary.

Turning the steering wheel rotates core valve 58 from its centeredposition. This closes some of the orifices in the fluid circuits 88, 90and opens the others. Flow through fluid circuit 88 is described below,it being understood that the description is also applicable to fluidcircuit 90.

From the perspective of FIG. 5, counterclockwise rotation of valve core58 closes orifices 124, 126, and 128 and opens orifices 130, 132, and134. Clockwise rotation closes orifices 130, 132, and 134 and opensorifices 124, 126, and 128. The closing orifices restrict flow. Thisgenerates a pressure imbalance that pressurizes one motor port 96 a or98 a and flows high-pressure fluid into the motor chamber connected tothe motor port. Fluid flows out of the other, low-pressure motor chamberand through the other motor port 98 a or 96 a. The orifices that open donot substantially impede or restrict flow. Flow for counterclockwiseoperation is described next, it being understood the description is alsoapplicable to clockwise rotation.

FIG. 6 illustrates valve core 58 rotated counterclockwise from thestraight-ahead, centered position to an operating position forpower-assisted steering. Control valve 10 is now in an off-centercondition as compared to the centered condition shown in FIG. 5.

Rotation constricts orifices 124, 126 and 128 and enlarges orifices 130,132 and 134. Orifice 130 is at the entrance of first flow passage 136and opens to permit essentially unimpeded flow from inlet port 92 a toflow passage 136. Pressure orifice 124 is at the entrance of second flowpassage 138 and restricts flow from inlet port 92 a to second flowpassage 138. As a result, flow from inlet port 92 a to first flowpassage 136 increases and flow to second flow passage 138 decreases.This pressurizes core groove 116 a and flows fluid out of first motorport 96 a to first motor chamber 38, moving piston 36 to the left asviewed in FIG. 2.

Piston 36 forces fluid from second motor chamber 40 through second motorport 98 a to second flow passage 138. Orifices 132 and 134 in secondflow passage 138 downstream of second motor port 98 a are open and thereturn flow is essentially unobstructed to exhaust port 94 b.

Note in FIG. 6 that the leading edge of core land 110 b is sufficientlyspaced away from sleeve land 100 d that no restrictive orifice formsbetween lands 110 b and 100 d with counterclockwise rotation of thevalve core 58. If a flow restriction is desired in flow passage 138downstream of second motor port 98 a, then lands 110 b and 100 d can besized to form such a flow restriction.

Upstream exhaust orifice 126 and downstream exhaust orifice 128 are inseries in first flow passage 136 downstream from the first motor port 96a as described. The orifices 126, 128 essentially control flow throughfirst motor port 96 a by generating a pressure in core groove 116 a.Orifices 126 and 128 close continuously with rotation until the valve 10reaches a fully closed condition.

Exhaust orifices 126, 128 are initially substantially open and generaterelatively low backpressure. Most of the flow in flow passage 136 flowsthrough orifices 126, 128, and only a relatively small of pressure isexerted on motor port 96 a. In this condition power assist is low. Asorifices 126, 128 close, pressure increases, pressure to motor port 96 aincreases, and power assist increases. At the fully closed condition,all flow (less leakage through closed orifices) from inlet port 92 aflows through motor port 96 a for maximum power assist.

The rate at which an orifice 124, 126, or 128 closes is a function ofthe circumferential spacing of the valve lands defining the orifice andthe shape of the valve land edges forming the orifice. Establishing theclosing rate, and changing the closing rate with rotation of the controlvalve by configuring the valve lands and the shape of the lands is knownin the art and so will not be described in detail.

FIGS. 7 a, 7 b, and 7 c are enlarged views of orifices 124, 126, and 128respectively. In the illustrated embodiment core land 110 a has auniform or constant outer radius that is slightly less than the innerradius of sleeve land 100 b. See FIG. 7 a. Core land 110 c includes acontrol edge 142 that faces sleeve land 100 a. See FIG. 7 b. Edge 142 isinclined at an angle 143 of 70.3 degrees from the radial as shown. Coreland 110 b includes a control edge 144 (see FIG. 5) like edge 142 butfacing sleeve land 100 b. Core land 114 a includes a control edge 146facing sleeve land 100 c. See FIG. 7 c. Edge 146 is inclined at an angle147 of 48.5 degrees from the radial as shown. Core land 114 b includes acontrol edge 148 (see FIG. 5) like control edge 146 but facing sleeveland 100 d.

Each control edge 142, 144, 146, and 148 is a flat surface extendingaxially along the land and formed as a chamfer or bevel on a corner ofthe land. In other embodiments the control edges could include curvedsurfaces, could be present exclusively on the sleeve lands, or could beon both sleeve and core lands.

FIG. 8 is a valve curve of the control valve 10. The curve plots thesize of orifices 124, 126, and 128 as a function of steering angle (therelative angular rotation of valve core 58 from the centered position).The size of the orifice is the minimum width of the orificecross-section perpendicular to the valve axis. Because the axial lengthof each orifice is much greater than its width, the effective flow areaof each orifice is essentially directly proportional to orifice size.

Pressure orifice 124 preferably closes quickly from the initial centeredcondition 150 of the valve 10. Core land 110 a is initially spaced awayfrom sleeve land 100 b to define the initial size of orifice 124.Orifice 124 closes as core land 110 a approaches sleeve land 10 b.Orifice 124 closes at a substantially constant rate 151 as shown. Theleading edge of core land 110 a comes under sleeve land 100 b after arotation of about 1.5 degrees, fully closing orifice 124. Thisrepresents a first operating condition 152 of the control valve 10.Quickly closing the pressure orifice 124 helps avoid “grump”, anunstable flow condition known in the hydraulic power steering art.

Upstream exhaust orifice 126 is initially the same size as the pressureorifice 124. Orifice 126 initially closes at a first closing rate 153substantially equal to the closing rate of the pressure orifice 124. Asthe control valve 10 reaches operating position 152, orifice 126 beginsclosing at a second, lower closing rate 154. Orifice 126 closes at thisessentially constant second closing rate until the control valve reachesa fully closed condition 156 at a rotation of about 4 degrees. Thecontrol valve 10 can continue to rotate beyond the fully closedposition.

In the illustrated embodiment the initial closing rate of upstreamexhaust orifice 126 is determined by the circumferential spacing of coreland 110 c from sleeve land 100 a. After control edge 142 moves undersleeve land 100 a, the slope of control edge 142 establishes the secondclosing rate. Other arrangements of land spacing and control edgeconfiguration, including control edges having multiple sloped surfaces,could be used in alternative embodiments.

Downstream exhaust orifice 128 closes with upstream exhaust orifice 126.Orifice 128 also fully closes when the control valve reaches fullyclosed valve condition 156, and closes at a substantially constantclosing rate 157 from the centered condition 150 to the fully closedcondition 156. The closing rate is established by the initial spacing ofcore land 114 a from sleeve land 100 c and the slope of control edge146. Downstream orifice 128 is larger than upstream exhaust orifice 126until both orifices fully close.

Between the first operating condition 152 and the fully closed condition156, downstream exhaust orifice 128 is about three times larger thanupstream exhaust orifice 126. Because orifices 126 and 128 both fullyclose at about 4 degrees, the closing rate of downstream exhaust orifice128 is also about three times greater than the closing rate of upstreamexhaust orifice 126 between operating conditions 152 and 154.

Exhaust orifices 126 and 128 cooperate to generate relatively lowerpower assist at small steering angles and greater power assist at largersteering angles for parking or sharp turns at low speed. Driversgenerally prefer power steering systems having these steeringcharacteristics.

FIG. 9 is a Wheatstone diagram of fluid circuits 88, 90 in control valve10 when the control valve in the off-center condition shown in FIG. 6.Fluid circuits 88, 90 are arranged in parallel between inlet passage 80and outlet passage 82. Orifices 130, 132 and 134 are omitted from thediagram because they open with increasing rotation.

A mathematical model of fluid circuits 88, 90 was used to arrive at theconfiguration of the upstream and downstream exhaust orifices 126, 128.The model calculates orifice sizes for a proposed configuration fordifferent steering angles. The calculated areas are used to calculatethe flow through the fluid circuits 88, 90 by solving simultaneousequations of flow through various portions of each circuit.

Sizes of orifices 126, 128 were determined by maximizing the cavitationnumbers through exhaust orifices 126, 128 for a fluid pressure of about1500 pounds per square inch and a flow rate of about 3.7 gallons perminute. This represents a typical output requirement for a heavy-dutytruck as opposed to full torque output. In the illustrated embodiment itis desired to minimize noise at a typical operating condition of thesteering system 12. Different operating conditions can be used asstarting points for control valve analysis in other embodiments, basedon engineering judgment and specific pressure and flow requirements.

For the illustrated embodiment it was determined that the size ofdownstream exhaust orifice 128 should be about three times the size ofupstream exhaust orifice 126 at the chosen operating condition. Thisreduces the perceived noise from control valve 10 in half as compared toa conventional single-stage control valve at the same operatingcondition.

The spacing of the core and sleeve lands associated with exhaustorifices 126 and 128, and the slope of control edges 142 and 146 wereselected to obtain the desired valve curve.

FIG. 10 is a representational view of the fluid circuits 88, 90extending around the valve core 58 when the valve core 58 in theoff-center position shown in FIG. 6. Each circuit 88, 90 extends 180degrees around the valve core 58. In circuit 88 pressure orifice 124 andupstream exhaust orifice 126 define between them a high pressure zone156 a. An intermediate pressure zone 158 a is defined between upstreamand downstream exhaust orifices 126 and 128. A low pressure zone 160 aextends from the downstream exhaust orifice 128 to the exhaust port 94a. Like high pressure zone 156 b, intermediate pressure zone 158 b, andlow pressure zone 160 b are in fluid circuit 90. The correspondingorifices and pressure zones in fluid circuit 90 have the same referencenumerals but are marked with primes.

The corresponding pairs of pressure zones 156, 158, and 160 arediametrically across from each other on valve core 58. As a result, thenet radial force acting on valve core 58 is theoretically zero.Stiction, or resistance to relative rotation, could occur between valvecore 58 and valve sleeve 56 when pressure in the intermediate zones 158were relatively high. It is believed such stiction would be caused by anet radial force applied against valve core 58 generated by a pressuredifference between intermediate zone 158 a and intermediate zone 158 b.

High pressure zones 156 a, 156 b are both fluidly connected to commonvalve inlet passage 80 by respective inlet ports 92 a, 92 b. Lowpressure zones 160 a, 160 b are both fluidly connected to common outletpassage 82 by respective exhaust ports 94 a, 94 b. The pressure in zones156 and 160 are established by the inlet pressure and exhaust pressurerespectively and so the pressure in the individual zones of each pair ofpressure zones 156 and 160 are equal with one another.

The pressure in each intermediate pressure zones 158 a or 158 b is afunction of the upstream and downstream exhaust orifices defining theintermediate pressure zone. Corresponding exhaust orifices in the twofluid circuits 88, 90 are theoretically identical with one another, andso the pressure in the intermediate pressure zones 158 a and 158 b wouldalso be theoretically equal.

Manufacturing tolerances, however, can cause one or both orifices in onefluid circuit 88 to be larger or smaller than the corresponding orificein the other fluid circuit 90.

As a result of these manufacturing tolerances, pressures in intermediatezones 158 a and 158 b are not likely to be equal. The pressuredifference generates a net radial force urging the valve core 58 to oneside. It is believed this force generates the observed stiction.

Equalization passage 120 fluidly connects intermediate pressure zones158 a and 158 b formed when valve core 58 is rotated counterclockwise.This equalizes pressure in both intermediate pressure zones despitevariations in orifice size. Equalization passage 122 fluidly connectsthe low pressure zones.

FIG. 11 is similar to FIG. 10 but illustrates fluid circuits 88, 90 whenvalve core 58 is rotated clockwise. Intermediate pressure zones 158 ineach circuit are fluidly connected by equalization passage 122.Equalization passage 120 fluidly connects the low pressure zones.Passages 120, 122 cooperate to effectively eliminate stiction of thecontrol valve 10 regardless of the direction the steering wheel isturned.

In the illustrated embodiment equalization passages 120, 122 arediametral through-bores contained entirely within valve core 58. Inother embodiments the equalization passages can be formed entirely orpartly in valve sleeve 56, or could include conduits that extend out ofthe valve housing to fluidly connect the intermediate pressure zones.

Control valve 10 defines a pair of fluid circuits 88, 90 arranged inparallel between the valve sleeve and the valve core to flow fluidbetween the control valve and the hydraulic motor 18. In otherembodiments additional fluid circuits could be defined that preferablyextend symmetrically around the valve core. Equalization passagesfunctionally equivalent to illustrated passages 120 and 122 wouldinterconnect the corresponding intermediate pressure zones of theadditional circuits to ensure equalized pressure as described.

Bearing cap 32 is conventional and includes a pressure relief cartridge162 (shown in FIG. 1) mounted on one side of the cap. A normally-closedpressure relief valve (not shown) is mounted in the cartridge 162 and islocated in a bypass circuit (not shown) that connects inlet passage 48and outlet passage 50 in parallel with the control valve 10. In theevent of an overpressure from the pump 14, the pressure relief valveopens. Fluid flows through the bypass circuit and bypasses the controlvalve 10. This limits the operating pressure of the steering gear andprevents an overpressure from being delivered to the hydraulic motor 18from control valve 10.

The location of cartridge 162 in bearing cap 32 can vary according tovehicle manufacturers' requirements. Moreover, some manufacturerseliminate the pressure relief valve and so cartridge 162 is omitted insome systems. As a result, a number of otherwise identical bearing caps32 must be manufactured and kept in inventory.

FIGS. 12–16 illustrate part of a second embodiment control valve 210 inaccordance with the present invention. Control valve 210 includes anouter valve member 212, like valve member 66, with valve sleeve 214 likevalve sleeve 56. Valve core 216 includes a pressure relief valve 218carried within it. By placing the pressure relief valve in the controlvalve 210, pressure relief cartridge 162 and its associated bypasscircuit can be eliminated from bearing cap 32.

Valve core 216 is similar to valve core 58 and so only the differenceswill be discussed. A bypass passage 220 extends through the valve core216 and connects the inlet ports with the exhaust ports in valve sleeve214. Bypass passage 220 is arranged in parallel with the fluid circuitscorresponding to fluid circuits 88, 90 during valve operation.

Pressure relief valve 218 is located in bypass passage 220 and includesa valving member 222. Illustrated valving member 222 is formed as asteel ball bearing but other shapes and materials for the valving membercan be used. Valving member 222 is movable between a closed positionagainst a valve seat 224 formed in the bypass passage and an openedposition away from the valve seat. Spring member 226, formed as a coilcompression spring, urges the valving member 222 against the valve seat224.

Bypass passage 220 includes a first bore 228 and a second bore 230 thateach extends into the valve core 216. Bore 228 is axially aligned withthe valve sleeve inlet ports and receives flow from the inlet ports.Bore 230 is axially spaced from bore 228 and is aligned with the valvesleeve exhaust ports to discharge flow to the exhaust ports. Bores 228and 230 are circuferentially offset 90 degrees from each other to beadjacent to the inlet and exhaust ports respectively.

A third bore 232 extends axially into the valve core 216 from steeringwheel end 234 and intersects first and second bores 228 and 230. Valveseat 224 is formed in bore 232 between bores 228 and 230. Blind bore 236extends axially into the valve core 216 from the opposite end of thevalve core to receive an end of the torsion bar in a conventionalmanner.

Spring 226 is captured in bore 232 between the valving member 222 and aplug 238 that carries O-ring seal 240 sealingly closing bore 232. Theaxial position of plug 238 can be varied as desired to change thepreload of spring 226 against the valving member 222. Spring 226 appliessufficient force against valving member 222 to maintain the pressurerelief valve in a closed condition under normal operating pressures.

In the event of an overpressure, fluid attempting to flow through bypasspassage 228 overcomes the force of spring 226. This moves valving member222 away from valve seat 224 and opens pressure relief valve 218. Flowthrough bypass passage 220 essentially bypasses the fluid circuitsdefined between valve sleeve 214 and valve core 216, thereby relievingan overpressure condition.

Pressure relief valve 218 can be modified or eliminated for differentpower steering systems without modifying bearing cap 32 and withoutmodifying other components of the power steering system. For example,the length, spring rate, or preload of spring 226 can be modified tochange the preload against valving member 222 for different operatingpressures without changing the size of valve sleeve 214 or valve core216. This enables identical bearing caps 32 to be used in differentpower steering systems and yet fully satisfy manufacturers'requirements.

The exhaust orifices defined in control valves 10, 210 extendsubstantially along the full axial length of the valve core landsdefining the orifices. Fluid between the exhaust orifices is essentiallycircumferential flow without a substantial axial flow component.

FIGS. 17–21 illustrate a third embodiment control valve 310 inaccordance with the present invention. Control valve 310 is similar tocontrol valve 10 and defines a pair of fluid circuits 312, 314 (see FIG.18) similar to fluid circuits 88, 90. The fluid in the intermediatepressure zones of control valve 10 does no work and is not discharged tothe hydraulic motor. This provides the opportunity in control valve 310to place additional flow restrictions in the intermediate pressurezones, without reducing pressure to the hydraulic motor and withoutincreasing backpressure in the hydraulic steering system. Theseadditional flow restrictions can be included in the mathematical modelpreviously described to optimize exhaust orifice areas and closingrates.

Control valve 310 includes an outer member 316 having a valve sleeve318, and a valve core 320. See FIG. 17. Valve sleeve 318 and valve core320 are similar to valve sleeve 56 and valve core 58, so only thedifferences will be discussed in detail.

As shown in FIG. 18, valve sleeve 318 includes lands 322 a, 322 b, 322c, and 322 d associated with fluid circuit 312 and lands 324 a, 324 b,324 c, and 324 d associated with fluid circuit 314. Lands 322 a–d and324 a–d are sized and spaced like lands 100 a–d and 102 a–d of valvesleeve 56.

Valve core 320 includes lands 326 a, 326 b, and 326 c associated withfluid circuit 312 and lands 328 a, 328 b, and 328 d associated withfluid circuit 314. Valve core lands 330 a and 330 b are shared betweenthe circuits. Lands 326 a-c, 328 a-c, and 330 a-b are sized and spacedlike respective lands 110 a-c, 112 a-c, and 114 a-b of valve core 58.

FIG. 18 illustrates control valve 310 in an off-center condition. Valvecore 320 is rotated counterclockwise with respect to valve sleeve 318 asviewed in the drawing. Lands 322 b and 326 a define pressure orifice332. Lands 322 a and 326 c define upstream exhaust orifice 334 and lands322 c and 330 b define downstream exhaust orifice 336. Counterclockwiserotation closes orifices 332, 334, and 336 in fluid circuit 312, andcorresponding orifices (not numbered) in fluid circuit 314. It isunderstood that clockwise rotation opens these orifices and closes theother pressure and exhaust orifices in the fluid circuits as previouslydescribed for control valve 10. The valve curve of control valve 310 issimilar to the valve curve of control valve 10.

Features of orifices 334 and 336 and in the intermediate pressure zonebetween the orifices will be discussed in detail below, it beingunderstood that the discussion is applicable to corresponding featuresfound in the other branch of fluid circuit 312 and both branches offluid circuit 314.

FIG. 19 illustrates valve core 320. Land 326 c has a control edge 338corresponding to control edge 142 of control valve 10. Control edge 338is adjacent the inner end of the valve core and extends axially only arelatively short distance along the land. This is unlike control edge142, which extends substantially the full length of the land. Controledge 338 defines a groove 339 in land 326 c that extendscircumferentially into the land.

Land 330 b has a control edge 340 corresponding to control edge 146 ofcontrol valve 10. Control edge 340 extends axially a relatively shortdistance along land 330 b and is axially spaced from control edge 338towards the steering wheel end of the control valve. Control edge 340defines a groove 341 in land 330 a that extends circumferentially intothe land.

Referring back to FIG. 18, orifices 334 and 336 essentially functionidentically like corresponding orifices 126 and 128 when control valve310 is in the centered condition. Pairs of sleeve/core lands 322 a/326 cand 322 c/330 b are spaced away from each other and flow throughorifices 334 and 336 is essentially circumferential along the entireaxial length of the lands.

When valve core 320 rotates to the off-center position shown, theleading edges of core lands 326 c and 330 b come beneath respectivesleeve lands 322 a and 322 c. Flow through orifice 334 is limited by theoverlapping lands to circumferential flow through groove 339, and flowthrough orifice 336 is limited by the overlapping lands tocircumferential flow through groove 341 (less any leakage betweenlands). Control edges 338 and 340 are configured such that the flowareas and closing rates of orifices 334 and 336 essentially equal theflow areas and closing rates of corresponding orifices 126 and 128 incontrol valve 10.

Sleeve groove 342 and core groove 344 cooperate to define an axial flowchannel 346 extending from orifice 334 to orifice 336 containing theintermediate pressure zone between orifices. Circumferential flowdischarged from orifice 334 is forced to flow axially through channel346 to orifice 336. Axial flow through channel 346 is forced back tocircumferential flow through orifice 336.

Changing the direction of flow between orifices 334 and 336 generatesadditional pressure drops in the intermediate pressure zone at thedischarge from orifice 334 and at the entrance to orifice 336. Theseadditional pressure drops are in series with the pressure drops throughexhaust orifices 334 and 336, and help reduce noise and the likelihoodof cavitation through the orifices.

Control valve 310 also includes an additional axial orifice 348 (seeFIG. 20) in the intermediate pressure zone described in greater detailbelow. Flow through orifice 348 generates an additional pressure dropthat further aids in reducing noise and the likelihood of cavitation incontrol valve 310.

As shown in FIG. 19, circumferential wall 350 extends between core lands326 c and 330 b. Wall 350 is about midway between control edges 338 and340 and blocks core groove 342. An axial slot 352 open at the top of thewall extends through the wall thickness. Valve sleeve 318 includes acircumferential wall 354 that extends between sleeve lands 322 a and 322c and blocks sleeve groove 346. See FIGS. 20 and 21.

Wall 354 overlays wall 350 and covers slot 352 to define the fixed-areaorifice 348. The walls 350, 354 divide the intermediate pressure zoneinto an upstream zone 356 receiving fluid from upstream exhaust orifice334 and a reduced-pressure downstream zone 358 flowing fluid todownstream exhaust orifice 336. An equalization passage interconnectsthe downstream zones in the two circuits, and an equalization passageinterconnecting the upstream zones can be provided.

Flow through channel 346 must pass through orifice 348, reducing fluidpressure and thereby reducing noise. The size and shape of orifice 348can be modified from the illustrated embodiment to achieve the desiredpressure drop and flow rate, and additional orifices 348 can be providedin channel 346.

In alternative embodiments of control valve 310, flow channel 346 canflow fluid radially between the exhaust orifices. For example, radialbores in valve core 320 having axially-spaced intake and discharge endscan be provided that flow fluid between the exhaust orifices. Flow mustchange radial direction, and the change in radial direction causesadditional pressure drop. The bores themselves can form or include flowrestrictions that also generate a pressure drop between exhaustorifices.

Illustrated control valves 10, 210 and 310 define a pair of fluidcircuits arranged in parallel between the valve sleeve and the valvecore to flow fluid between the control valve and the hydraulic motor. Inyet other embodiments of the present invention the inner and outer valvemembers can define only a single fluid circuit within the control valve.Such control valves are generally not preferred for power steeringapplications because of their inherent load imbalance, but could bepractical in other applications.

Control valves in accordance with the present invention can also be usedin rack-and-pinion power steering systems or to control flow tohydraulic components in other types of hydraulic systems.

Control valves in accordance with the present invention are preferablyformed as rotary control valves, with inner and outer valve membersco-axially mounted for relative rotation about an axis of rotation.Other possible embodiments of control valves in accordance with thepresent invention could be formed as longitudinal control valves, inwhich the inner and outer valve members are co-axially mounted forrelative translation along an axis.

While we have illustrated and described preferred embodiments of ourinvention, it is understood that this is capable of modification, and wetherefore do not wish to be limited to the precise details set forth,but desire to avail ourselves of such changes and alterations as fallwithin the purview of the following claims.

1. A hydraulic steering system for turning steerable wheels of avehicle, the system comprising: an output member movable to effectturning the steerable wheels of the vehicle; a hydraulic motor formoving the output member, the motor having first and second fluidchambers; a pump for supplying fluid to the motor from a fluid source; acontrol valve for controlling fluid flow between the pump and the motor;and conduits providing fluid communication between the motor and thepump and the control valve; the control valve comprising an outer valvemember and an inner valve member, the inner and outer valve memberssupported in a housing for relative rotation about an axis between acentered condition of the control valve and a closed condition of thecontrol valve, means for biasing the inner and outer valve members tothe centered condition, an inlet passage for flowing fluid from the pumpinto the control valve and an exhaust passage for flowing fluid out ofthe control valve to the fluid source; the outer valve member comprisingan inner periphery and the inner valve member comprising an outerperiphery adjacent to the inner periphery of the outer valve member, theinlet passage flowing fluid to the inner periphery of the outer valvemember, the exhaust passage flowing fluid from the inner periphery ofthe outer valve member; the inner and outer peripheries of the valvemembers defining first and second flow passages between the valvemembers, said flow passages arranged in parallel and fluidly connectingthe inlet passage and the exhaust passage; the control valve furthercomprising a first motor passage extending from the first flow passagefor flowing fluid between the first flow passage and the first motorchamber, and a second motor passage extending from the second flowpassage for flowing fluid between the second flow passage and the secondmotor chamber; the inner periphery of the outer valve member and theouter periphery of the inner valve member defining a plurality of floworifices, each orifice being variable in flow area in response torelative rotation of the inner valve member in a first direction fromthe centered condition to the closed condition; the plurality oforifices comprising a pressure orifice in the second flow passagebetween the inlet passage and the second motor passage, the pressureorifice closing upon movement of the control valve in the firstdirection to flow fluid from the control valve to the first motorchamber and to flow fluid from the second motor chamber to the controlvalve for movement of the steering member; the plurality of orificescomprising first and second exhaust orifices in series in the first flowpassage, the first exhaust orifice between the first motor passage andthe exhaust passage, the second exhaust orifice between the firstexhaust orifice and the exhaust passage; each of the first and secondexhaust orifices closing upon movement of the control valve in the firstdirection from the centered condition to the closed condition, theexhaust orifices closing to an essentially fully closed condition atabout the same movement of the control valve from the centered conditionto the closed condition; and the second exhaust orifice having asubstantially larger flow area than the first exhaust orifice when thecontrol valve is not in the closed position.
 2. The hydraulic steeringsystem of claim 1 wherein the pressure orifice closes to an essentiallyfully closed condition substantially before the first exhaust orificecloses as the control valve moves from the centered condition to theclosed condition.
 3. The hydraulic steering system of claim 1 whereinthe second exhaust orifice closes at a substantially constant rate asthe control valve moves from the centered condition to the closedcondition.
 4. The hydraulic steering system of claim 1 wherein: theinner valve member is relatively rotatable in the first direction fromthe centered condition to a first operating condition of the controlvalve, and from the first operating condition to the closed condition ofthe control valve; and the first exhaust orifice closes relativelyrapidly as the control valve moves from the centered condition to thefirst operating condition and closes relatively slowly as the controlvalve moves from the first operating condition to the closed condition.5. The hydraulic steering system of claim 4 wherein: the first exhaustorifice closes at a substantially constant first rate as the controlvalve moves from the centered condition to the first operating conditionand closes at a substantially constant second rate as the control valvemoves from the first operating condition to the closed condition.
 6. Thehydraulic steering system of claim 4 wherein: the first and secondexhaust orifices each close at a respective substantially constant rateas the control valve moves from the first operating condition to theclosed condition.
 7. The hydraulic steering system of claim 6 whereinthe closing rate of the second exhaust orifice is approximately threetimes greater than the closing rate of the first exhaust orifice.
 8. Thehydraulic steering system of claim 4 wherein the pressure orifice is inan essentially fully closed condition when the control valve is in thefirst operating condition.
 9. The hydraulic steering system of claim 1wherein the pressure orifice is effectively the sole orifice in thesecond flow passage as the control valve is moved from the centeredcondition to the closed condition.
 10. The hydraulic steering system ofclaim 1 wherein said periphery of each of the inner and outer valvemembers comprises a plurality of lands and grooves, the lands andgrooves of the valve members cooperating to form said orifices.
 11. Thehydraulic steering system of claim 10 wherein the plurality of lands andgrooves of the inner valve member comprises a first land associated withthe first exhaust orifice and a second land associated with the secondexhaust orifice; and each of the first and second lands comprises arespective control edge for varying the flow area of the orificeassociated with the land.
 12. The hydraulic steering system of claim 11wherein each control edge comprises a substantially flat surface. 13.The hydraulic steering system of claim 12 wherein the surface associatedwith the first exhaust orifice is inclined at a first inclination angleand the surface associated with the second exhaust orifice is inclinedat a second inclination angle substantially different from the firstangle.
 14. The hydraulic steering system of claim 1 wherein said flowpaths, said orifices and said motor passages represent a fluid circuit,and the control valve comprises a plurality of like fluid circuitsarranged in parallel fluidly connecting the inlet passage and the returnpassage.
 15. The hydraulic steering system of claim 14 wherein theplurality of fluid circuits comprise a first fluid circuit and a secondfluid circuit, the first and second exhaust orifices of each of saidfirst and second fluid circuits defining between them a portion of thefirst flow path of such circuit; and the control valve comprises anadditional flow passage fluidly connecting said flow path portionswhereby the fluid pressures in the flow path portions are substantiallyequalized with each other.
 16. The hydraulic steering system of claim 1wherein the control valve comprises a pressure relief valve mounted inthe inner valve member.
 17. The hydraulic steering system of claim 1wherein the inner valve member is rotatable from the centered conditionto an off-center condition of the control valve; and the control valvecomprises structure defining a flow restriction in the first flowpassage between the first and second exhaust orifices when the innervalve member is in the off-center condition; whereby the inlet pressureat the second exhaust orifice is reduced from the discharge pressure atthe first exhaust orifice by the flow restriction.
 18. A control valvesuitable for controlling a fluid motor having first and second fluidchambers, the valve comprising: a housing; an outer valve member and aninner valve member, the inner and outer valve members supported in thehousing for relative motion in a first direction from a neutralcondition of the control valve and a closed condition of the controlvalve; an inlet passage for flowing fluid into the control valve and anexhaust passage for flowing fluid out of the control valve; the innerand outer valve members arranged to define first and second flowpassages between the valve members, the first and second flow passagesarranged in parallel and fluidly connecting the inlet passage and theexhaust passage; the control valve further comprising a first motorpassage extending from the first flow passage for flowing fluid betweenthe first flow passage and the first motor chamber, and a second motorpassage extending from the second flow passage for flowing fluid betweenthe second flow passage and the second motor chamber; the inner andouter valve members cooperating to define a pressure orifice, a firstexhaust orifice, and a second exhaust orifice in the first and secondflow passages, each orifice being variable in flow area in response tomotion of the inner valve member in the first direction; the pressureorifice in the second flow passage between the inlet passage and thesecond motor passage, the first and second exhaust orifices in series inthe first flow passage, the first exhaust orifice between the firstmotor passage and the exhaust passage, the second exhaust orificebetween the first exhaust orifice and the exhaust passage; each of thefirst and second exhaust orifices closing upon movement of the controlvalve in the first direction, the exhaust orifices closing to anessentially fully closed condition at about the same movement of thecontrol valve from the neutral condition to the closed condition; andthe second exhaust orifice having a substantially larger flow area thanthe first exhaust orifice when the first and second orifices are betweenthe neutral and closed conditions.
 19. The control valve of claim 18wherein the pressure orifice closes to an essentially fully closedcondition substantially before the first exhaust orifice closes as thecontrol valve moves from the neutral position to the closed position.20. The control valve of claim 18 wherein the second exhaust orificecloses at a substantially constant rate as the control valve moves fromthe neutral condition to the closed condition.
 21. The control valve ofclaim 18 wherein: the inner valve member is relatively movable in thefirst direction from the neutral condition to a first operatingcondition of the control valve, and from the first operating conditionto the closed condition of the control valve; and the first exhaustorifice closes relatively rapidly as the control valve moves from theneutral condition to the first operating condition and closes relativelyslowly as the control valve moves from the first operating condition tothe closed condition.
 22. The control valve of claim 21 wherein: thefirst exhaust orifice closes at a substantially constant first rate asthe control valve moves from the neutral condition to the firstoperating condition and closes at a substantially constant second rateas the control valve moves from the first operating condition to theclosed condition.
 23. The control valve of claim 21 wherein: the firstand second exhaust orifices each close at a respective substantiallyconstant rate as the control valve moves from the first operatingcondition to the closed condition.
 24. The control valve of claim 23wherein the closing rate of the second exhaust orifice is approximatelythree times greater than the closing rate of the first exhaust orifice.25. The control valve of claim 21 wherein the pressure orifice is in anessentially fully closed condition when the control valve is in thefirst operating condition.
 26. The control valve of claim 18 wherein thesecond flow passage is essentially unobstructed between the pressureorifice and the exhaust passage as the control valve is moved from theneutral condition to the closed condition.
 27. The control valve ofclaim 18 wherein the inner and outer valve members are mounted forrelative rotation about an axis of rotation.
 28. The control valve ofclaim 18 wherein said flow paths, said orifices and said motor passagesrepresent a fluid circuit, and the control valve comprises a pluralityof like fluid circuits arranged in parallel fluidly connecting the inletpassage and the return passage.
 29. The control valve of claim 28wherein the plurality of fluid circuits comprise a first fluid circuitand a second fluid circuit, the first and second exhaust orifices ofeach of said first and second fluid circuits defining between them aportion of the first flow path of such circuit; and the control valvecomprises an additional flow passage fluidly connecting said flow pathportions whereby the fluid pressures in the flow path portions aresubstantially equalized with each other.
 30. The control valve of claim18 comprising a pressure relief valve within the housing.
 31. Thecontrol valve of claim 18 wherein the control valve comprises structuredefining a flow restriction in the first flow passage between the firstand second exhaust orifices when the inner valve member is in anoff-center condition whereby the inlet pressure at the second exhaustorifice is reduced from the discharge pressure at the first exhaustorifice by the flow restriction.
 32. A hydraulic steering system forturning steerable wheels of a vehicle, the system comprising: an outputmember movable to effect turning the steerable wheels of the vehicle; ahydraulic motor for moving the output member, the motor having first andsecond fluid chambers; a pump for supplying fluid to the motor from afluid source; a control valve for controlling fluid flow between thepump and the motor; and conduits providing fluid communication betweenthe motor and the pump and the control valve; the control valvecomprising an outer valve member and an inner valve member, the innerand outer valve members supported in a housing for relative rotationabout an axis from a centered condition of the control valve, means forbiasing the inner and outer valve members to the centered condition, aninlet passage for flowing fluid from the pump into the control valve andan exhaust passage for flowing fluid out of the control valve to thefluid source; the outer valve member comprising an inner periphery andthe inner valve member comprising an outer periphery adjacent to theinner periphery of the outer valve member, the inlet passage flowingfluid to the inner periphery of the outer valve member, the exhaustpassage flowing fluid from the inner periphery of the outer valvemember; the inner and outer peripheries of the valve members definingfirst and second fluid circuits between the valve members, said fluidcircuits arranged in parallel and fluidly connecting the inlet passageand the exhaust passage; each fluid circuit comprising first and secondflow passages arranged in parallel and fluidly connecting the inletpassage and the exhaust passage, a first motor passage for flowing fluidbetween the first flow passage and the first motor chamber, and a secondmotor passage for flowing fluid between the second flow passage and thesecond motor chamber; the inner periphery of the outer valve member andthe outer periphery of the inner valve member defining a first pluralityof flow orifices, each orifice being variable in flow area in responseto relative rotation of the inner valve member in a first direction fromthe centered condition of the control valve; the first plurality oforifices comprising a pressure orifice in each second flow passagebetween the inlet passage and the second motor passage of the respectivesecond flow passage, the pressure orifice closing upon movement of thecontrol valve in a first direction from the centered condition to flowfluid from the control valve to the first motor chamber and to flowfluid from the second motor chamber to the control valve for movement ofthe steering member; the first plurality of orifices comprising firstand second exhaust orifices in series in each first flow passage, thefirst and second exhaust orifices between the first motor passage andthe exhaust passage of the respective first flow passage, the first andsecond exhaust orifices defining between them a flow path portion ofsuch circuit; and the control valve comprises an equalization passagefluidly connecting the flow path portions of the first and second fluidcircuits whereby the fluid pressures in the flow path portions aresubstantially equalized with each other despite variations in orificesizes caused by manufacturing tolerances.
 33. The control valve of claim32 wherein the equalization passage is contained entirely within thehousing.
 34. The control valve of claim 32 wherein the equalizationpassage is formed in the inner valve member.
 35. The control valve ofclaim 34 wherein the equalization passage comprises a bore extendingthrough the inner valve member, the bore comprising openings at oppositeends of the bore on the outer periphery of the inner valve member. 36.The control valve of claim 35 wherein the bore extends along a diameterof the inner valve member.
 37. The control valve of claim 32 wherein theequalization passage represents a first equalization passage and theflow path portions represent first flow path portions; the innerperiphery of the outer valve member and the outer periphery of the innervalve member define a second plurality of flow orifices, each orifice ofthe second plurality of orifices being variable in flow area in responseto relative rotation of the inner valve member in a second directionfrom the centered condition of the control valve; the second pluralityof orifices comprising a pressure orifice in each first flow passagebetween the inlet passage and the first motor passage of the respectivefirst flow passage, the pressure orifice closing upon movement of thecontrol valve in a second direction from the centered condition to flowfluid from the control valve to the second motor chamber and to flowfluid from the first motor chamber to the control valve for movement ofthe steering member; the second plurality of orifices comprising firstand second exhaust orifices in series in each second flow passage, thefirst and second exhaust orifices between the second motor passage andthe exhaust passage of the respective second flow passage, the first andsecond exhaust orifices defining between them a second flow portion ofsuch circuit; and the control valve comprises a second equalizationpassage fluidly connecting the second flow path portions of the firstand second fluid circuits whereby the fluid pressures in the second flowpath portions are substantially equalized with each other despitevariations in orifice sizes caused by manufacturing tolerances.
 38. Thecontrol valve of claim 37 wherein the inner valve member extends alongan axis and the first and second equalization passages are axiallyspaced from each other.
 39. The control valve of claim 37 wherein thefirst and second equalization passages are each contained within thehousing.
 40. The control valve of claim 39 wherein the first and secondequalization passages are each formed in the inner valve member.
 41. Acontrol valve suitable for controlling a hydraulic motor having firstand second fluid chambers, the valve comprising: a housing; an outervalve member and an inner valve member, the inner and outer valvemembers supported in the housing for relative motion in a firstdirection from a neutral position of the control valve; an inlet passagefor flowing fluid into the control valve and an exhaust passage forflowing fluid out of the control valve; the inner and outer valvemembers arranged to define first and second flow circuits between thevalve members, the flow circuits arranged in parallel and fluidlyconnecting the inlet passage and the exhaust passage; each flow circuitcomprising first and second flow passages arranged in parallel andfluidly connecting the inlet passage and the exhaust passage, a firstmotor passage for flowing fluid between the first flow passage and thefirst motor chamber, and a second motor passage for flowing fluidbetween the second flow passage and the second motor chamber; the innerand outer valve members cooperating to define a pressure orifice, afirst exhaust orifice, and a second exhaust orifice in the first andsecond flow passages of each flow circuit, each orifice being variablein flow area in response to motion of the inner valve member in thefirst direction; the pressure orifice in each second flow passagebetween the inlet passage and the second motor passage of the secondflow passage, the first and second exhaust orifices in each first flowpassage in series between the first motor passage of the first flowpassage and the exhaust passage, the first and second exhaust orificesdefining between them a flow path portion of the flow circuit containingthe first and second exhaust orifices; and the control valve comprisesan equalization passage fluidly connecting the flow path portions of thefirst and second flow circuits whereby the fluid pressures in the flowpath portions are substantially equalized with each other despitevariations in orifice sizes caused by manufacturing tolerances.
 42. Thecontrol valve of claim 41 wherein the equalization passage is containedentirely within the housing.
 43. The control valve of claim 41 whereinthe equalization passage represents a first equalization passage and theflow path portions represent first flow path portions; the innerperiphery of the outer valve member and the outer periphery of the innervalve member define a second plurality of flow orifices, each orifice ofthe second plurality of orifices being variable in flow area in responseto relative rotation of the inner valve member in a second directionfrom the neutral condition of the control valve; the second plurality oforifices comprising a pressure orifice in each first flow passagebetween the inlet passage and the first motor passage of the respectivefirst flow passage, the pressure orifice closing upon movement of thecontrol valve in a second direction from the neutral condition to flowfluid from the control valve to the second motor chamber and to flowfluid from the first motor chamber to the control valve for movement ofthe steering member; the second plurality of orifices comprising firstand second exhaust orifices in series in each second flow passage, thefirst and second exhaust orifices between the second motor passage andthe exhaust passage of the respective second flow passage, the first andsecond exhaust orifices defining between them a second flow portion ofsuch circuit; and the control valve comprises a second equalizationpassage fluidly connecting the second flow path portions of the firstand second fluid circuits whereby the fluid pressures in the second flowpath portions are substantially equalized with each other despitevariations in orifice sizes caused by manufacturing tolerances.
 44. Thecontrol valve of claim 43 wherein the inner valve member extends alongan axis and the first and second equalization passages are axiallyspaced from each other.
 45. The control valve of claim 43 wherein thefirst and second equalization passages are each contained within thehousing.
 46. A control valve suitable for controlling a fluid motorhaving first and second fluid chambers, the valve comprising: a housing;an outer valve member and an inner valve member the inner and outervalve members supported in the housing for relative motion to operatethe valve; an inlet passage for flowing pressurized fluid into thecontrol valve and an exhaust passage for flowing fluid out of thecontrol valve; the inner and outer valve members arranged to define aflow passage between the valve members, the flow passage fluidlyconnecting the inlet passage and the exhaust passage for flowing fluidthrough the valve; the control valve further comprising first and secondmotor passages extending from the flow passage, the first motor passagefor flowing fluid between the flow passage and the first fluid chamber,the second motor passage for flowing fluid between the flow passage andthe second motor chamber, and a bypass passage in the inner valvemember, the bypass passage arranged in parallel with the flow passageand fluidly connecting the inlet passage and the exhaust passage; and apressure relief valve in the inner valve member, the pressure reliefvalve comprising a valving member in the bypass passage, the valvingmember movable between opened and normally closed positions to open andclose the pressure relief valve, and means for biasing the valvingmember to the closed position; the valving member arranged such thatfluid attempting to flow through the bypass passage urges the valvingmember to the opened position against the biasing means so that anoverpressure opens the pressure relief valve to flow fluid through thebypass passage.
 47. The control valve of claim 46 wherein the pressurerelief valve comprises a valve seat formed in the bypass passage, thevalving member against the valve seat when in the closed position. 48.The control valve of claim 47 wherein the means for biasing the valvingmember comprises a spring in the bypass passage.
 49. The control valveof claim 46 wherein the bypass passage comprises a first bore, a secondbore, and a third bore, the first bore extending from the inlet into theinner valve member, the second bore spaced from the first bore andextending from the discharge into the inner valve member, and the thirdbore intersecting the first and second bores.
 50. The control valve ofclaim 49 wherein the pressure relief valve is located in the third bore.51. The control valve of claim 50 wherein the third bore comprises anopen end portion at an end of the inner valve member, and the pressurerelief valve comprises a plug sealingly closing the open end portion ofthe third bore.
 52. The control valve of claim 51 wherein the biasingmeans comprises a coil spring compressed between the plug and the valvemember generating a spring force urging the valve member towards thevalve seat, and the plug is movable along the third bore to adjust thespring force.
 53. The control valve of claim 46 wherein the inner andouter valve members cooperatively define at least one variable floworifice in the flow passage.
 54. The control valve of claim 53 whereinthe inner and outer valve members are relatively movable about an axisof rotation.
 55. The control valve of claim 46 comprising means forbiasing the first and second valve members to a neutral position of thecontrol valve.
 56. The control valve of claim 55 wherein the means forbiasing the first and second valve members comprises a torsion baroperatively connected between the inner and outer valve members.
 57. Avalve subassembly for a hydraulic control valve that controls the flowof fluid between the valve and a fluid machine, the subassemblycomprising: a valve member and a pressure relief valve in the valvemember, the valve member configured for relative motion within a secondvalve member of the control valve to operate the control valve; thevalve member comprising first, second, and third surfaces, the firstsurface configured to at least partially define an inlet passage forflowing fluid into the control valve when the valve member is in thecontrol valve, the second surface configured to at least partiallydefine an exhaust passage for flowing fluid out of the control valvewhen the valve member is in the control valve, and the third surface onan outer periphery of the valve member and configured to define a flowpassage fluidly connecting the inlet and exhaust passages when the valvemember is in the control valve to flow fluid between the control valveand the hydraulic machine; the valve member comprising a bypass passageextending through the valve member, the bypass-passage comprising aninlet opening on the first surface and a discharge opening on the secondsurface, the bypass passage configured to fluidly connect the inlet andexhaust passages in parallel with the flow passage when the valve memberis in the control valve; the pressure relief valve comprising a valvingmember in the bypass passage, the valving member movable between openedand normally closed positions to open and close the bypass passage, andmeans for biasing the valving member to the closed position; the valvingmember arranged such that fluid attempting to flow through the bypasspassage urges the valving member to the opened position whereby anoverpressure condition opens the pressure relief valve and fluid flowingthrough the control valve bypasses the flow passage.
 58. The valveassembly of claim 57 wherein the valve member extends along an axis andthe first and second surfaces are axially adjacent each other.
 59. Thevalve assembly of claim 58 wherein the bypass passage comprisesaxially-spaced first and second bores extending into the valve member,the first bore comprising the inlet opening and the second borecomprising the discharge opening.
 60. The valve assembly of claim 59wherein the bypass passage comprises a third bore in the valve member,the third bore intersecting the first and second bores, and the pressurerelief valve comprises a valve seat in the third bore between the firstand second bores.
 61. The valve assembly of claim 60 wherein the thirdbore is open at an end of the valve member, the pressure relief valvecomprises a plug in the third bore sealingly closing the open end of thethird bore, and the means for biasing the valving member comprises aspring in the third bore between the plug and the valving member. 62.The valve assembly of claim 61 wherein the spring comprises a compressedlength between the plug and the valving member when the valving memberis in the closed position, and the plug is adjustably mounted in thethird bore for movement towards and away from the valving member foradjustment of the compressed length of the spring.
 63. The valveassembly of claim 57 wherein the control valve is of the type having atorsion bar to bias the valve members to a neutral position, the valvemember comprising means for attaching the torsion bar to the valvemember.
 64. The valve assembly of claim 57 wherein the third surface ofthe valve member comprises at least one flow control surface configuredto cooperate with the second valve member to form a variable-floworifice in the fluid path when the valve member is in the control valve.65. The valve assembly of claim 64 wherein the outer periphery of thevalve member comprises a groove and a land adjacent the groove, the flowcontrol surface on the land.
 66. A control valve suitable forcontrolling a fluid motor having first and second fluid chambers, thevalve comprising: a housing; an outer valve member and an inner valvemember, the inner and outer valve members supported in the housing forrelative motion in a first direction from a neutral condition of thecontrol valve and a closed condition of the control valve; an inletpassage for flowing fluid into the control valve and an exhaust passagefor flowing fluid out of the control valve; the inner and outer valvemembers arranged to define first and second flow passages between thevalve members, the first and second flow passages arranged in paralleland fluidly connecting the inlet passage and the exhaust passage; thecontrol valve further comprising a first motor passage extending fromthe first flow passage for flowing fluid between the first flow passageand the first motor chamber, and a second motor passage extending fromthe second flow passage for flowing fluid between the second flowpassage and the second motor chamber; the inner and outer valve memberscooperating to define a pressure orifice, a first exhaust orifice, and asecond exhaust orifice in the first and second flow passages, eachorifice being variable in flow area in response to motion of the innervalve member in the first direction; the pressure orifice in the secondflow passage between the inlet passage and the second motor passage, thefirst and second exhaust orifices in series in the first flow passage,the first exhaust orifice between the first motor passage and theexhaust passage, the second exhaust orifice between the first exhaustorifice and the exhaust passage, the first flow path between the firstand second exhaust orifices entirely within the housing; and a flowrestriction in the first flow path between the first and second exhaustorifices, the flow restriction configured to reduce the pressure of thefluid discharged from the first exhaust orifice before entering thesecond exhaust orifice.
 67. The hydraulic steering system of claim 66wherein the inner valve member is rotatable from the centered conditionto an off-center condition of the control valve; and the control valvecomprises structure defining a flow restriction in the first flowpassage between the first and second exhaust orifices when the innervalve member is in the off-center condition; whereby the pressure of thefluid discharged from the first exhaust orifice is reduced by the flowrestriction before entering the second exhaust orifice.
 68. Thehydraulic steering system of claim 66 wherein the flow restrictioncomprises at least one additional flow orifice between the first andsecond exhaust orifices.
 69. The hydraulic steering system of claim 68wherein the at least one additional flow orifice has a fixed flow area.70. The hydraulic steering system of claim 66 wherein the first andsecond exhaust orifices each flows fluid in substantially a firstdirection through the orifice; and the flow restriction structure flowsfluid between the first and second exhaust orifices in a seconddirection substantially different from the first direction to reduce thepressure of the fluid discharged from the first exhaust orifice beforeit enters the second exhaust orifice.
 71. The hydraulic steering systemof claim 70 wherein the first direction of flow is substantiallycircumferential about the axis and the second direction of flowcomprises a substantially axial component, substantially radialcomponent, or substantially radial-axial component with respect to theaxis.
 72. The hydraulic steering system of claim 66 wherein theperiphery of each of the inner and outer valve members comprises aplurality of lands, each land having an axial length; the first exhaustorifice is defined by respective first lands of the inner and outervalve members and the second exhaust orifice is defined by respectivesecond lands of the inner and outer valve members, the first landscomprising a first control edge and the second lands comprising a secondcontrol edge, each control edge having an axial dimension substantiallyless than the axial length of the lands associated with the controledge; the first lands are circumferentially spaced from each other andthe second lands are circumferentially spaced from each other when thecontrol valve is in the centered condition; the first lands are in anoverlapped condition and the second lands are in an overlapped conditionwhen the control valve is in the off-center condition; the flowrestriction structure comprises the outer periphery of the lands beingconfigured to sealing overlap and substantially close the overlap awayfrom the control edges when the control valve is in the off-centercondition; and the flow restriction structure comprises axially spacingthe first control edge from the second control edge whereby the firstflow passage extends axially between the first and second exhaustorifices when the control valve is in the off-center condition.
 73. Thehydraulic steering system of claim 72 wherein the flow restrictionstructure comprises a wall substantially closing the first flow passagebetween the first and second flow orifices when the control valve is inthe off-center condition and an additional flow orifice extendingthrough the wall fluidly connecting the first and second exhaustorifices.
 74. The hydraulic steering system of claim 73 wherein flowthrough the additional flow orifice includes an axial flow component, aradial flow component, or a radial-axial flow component with respect tothe axis.
 75. A method of operating a control valve in a power steeringsystem of a wheeled vehicle, the method comprising the steps of: (A)supplying working fluid to an inlet passage of the control valve andbetween inner and outer valve members; (B) moving the inner valve memberrelative to the outer valve member, the member, the movementrepresenting movement of the control valve towards a closed condition ofthe control valve; (C) flowing fluid through first and second flowpassages formed between the valve members, the first and second flowpassages arranged in parallel and fluidly connecting the inlet passagewith an exhaust passage that exhausts fluid from the control valve; (D)forming a pressure orifice in the first flow passage in response torelative movement of the inner valve member; (E) forming a first exhaustorifice and a second exhaust orifice in the second flow passage inresponse to the relative movement of the inner valve member, the secondexhaust orifice downstream from the first exhaust orifice; (F) flowingfluid to the first flow passage through a first motor passage fluidlyconnecting the first flow passage to a hydraulic motor, the first motorpassage downstream from the pressure orifice; (G) flowing fluid from thesecond flow passage through a second motor passage fluidly connectingthe second flow passage to the hydraulic motor, the second flow passageupstream from the first exhaust oriface; (H) closing the first andsecond exhaust orifaces as the control valve moves from the neutralcondition towards the closed condition; and (I) maintaining the flowarea of the second exhaust oriface greater than the flow area of thefirst exhaust oriface as the inner valve member moves from the neutralconditions towards the closed condition.
 76. The method of claim 75comprising the step of: (J) fully closing the pressure orifice and thefirst exhaust orifice when the control valve is in its closed condition,wherein the pressure orifice closes to a fully closed conditionsubstantially before the first exhaust orifice completely closes as thecontrol valve moves to the closed condition.
 77. The method of claim 75comprising the steps of: (J) moving the inner valve member relative tothe outer valve member to a first operating condition of the controlvalve and moving the inner valve member relative to the outer valvemember from the first operating condition to the closed condition; (K)closing the first exhaust orifice relatively quickly as the controlvalve moves from the neutral condition to the first operating condition;and (L) closing the first exhaust orifice relatively slowly as thecontrol valve moves from the first operating condition to the closedcondition.
 78. The method of claim 77 wherein step (K) comprises thestep of closing the first exhaust orifice at a substantially constantfirst closing rate and step (L) comprises the step of closing the firstexhaust orifice at a substantially constant second closing rate.
 79. Themethod of claim 77 comprising the step of: (M) closing each of the firstand second exhaust orifices at a respective substantially constant rateas the control valve moves from the first operating condition to theclosed condition.
 80. The method of claim 79 wherein the closing rate ofthe second exhaust orifice is about three times greater than the closingrate of the first exhaust orifice as the control valve moves from thefirst operating condition to the closed condition.
 81. The method ofclaim 77 wherein the pressure orifice is in essentially a fully closedcondition when the control valve is in the first operating condition.82. The method of claim 75 wherein the second flow passage isessentially without obstructions between the pressure orifice and theexhaust passage as the control valve moves from the neutral condition tothe closed condition.
 83. The method of claim 75 wherein step (B)comprises relative rotation of the inner valve member with respect tothe outer valve member about an axis of rotation.
 84. The method ofclaim 75 wherein said flow paths, said orifices, and said motor passagesrepresent a fluid circuit, and the control valve comprises a pluralityof like fluid circuits arranged in parallel fluidly connecting theintake and exhaust passages.
 85. The method of claim 84 comprising thestep of: (J) fluidly connecting the portions of the second flow pathsextending between the first and second exhaust orifices to equalize thefluid pressure in said portions.
 86. The method of claim 75 comprisingthe step of: (J) flowing fluid through an additional flow restrictionbetween the first and second exhaust orifices to further reduce thepressure of fluid discharged from the first exhaust orifice prior to thefluid reaching the second exhaust orifice.